1. Field of the Invention
The present invention generally relates to the public switched telephone network signaling system seven, SS7 and more particularly to a system and method for bypassing the SS7 network when forwarding SS7 messages between subsystems located at the same signaling control point.
2. Background of the Invention
The public switched telephone network, or PSTN, as we know it today was developed to allow telephone calls to be made to and from points anywhere in the world. To make this possible, standards had to be developed. One of the most important was the signaling system seven, or SS7 which controls the signaling needed to set up calls. The SS7 standard was originally developed to allow signaling for large numbers of calls to be sent over a small number of telephone lines, thereby reserving more lines for the voice connections. However, the SS7 standard has facilitated the development of many other functions on the PSTN, such as 800 service, 900 service, 911 service, mobile telephone service, and position determination service for mobile telephones.
The original PSTN with the SS7 was centered around telephone switches. The switches were essentially hardwired systems which used the signaling information from the SS7 system to build the connections between two or more telephone sets. The switches were not well suited for “non-standard” functions such as 800 service and were difficult to modify.
The inflexibility of the SS7 switches was addressed by adding service control points, SCPs, to the PSTN. Each SCP is identified by a signaling point code, SPC, often referred to as simply the point code. The SCPs often were essentially databases needed, for example, to convert an 800 number to a standard phone number which a switch can use to make the desired connection. When a switch received an 800 number, it would simply forward the SS7 message to the point code of the SCP providing 800 service. The SCP would look up the standard phone number and, using an SS7 message, return it to the switch which then completed the call.
As the number and complexity of telephone services increased, the SCPs were upgraded to include more intelligence. Many SCPs now are computer servers and the services or functions are controlled entirely by computer programs. An intelligent SCP may be variously referred to as an intelligent network server, INS, an intelligent peripheral, IP, or a services node, SN. An intelligent SCP typically contains a number of functions or services which all therefore are identified by the same point code. Each service application is also identified by a subsystem number, SSN. A SS7 message to a particular network service therefore contains both the point code and a SSN. All of the SCPs follow the SS7 protocol so that messages can be sent to and from any of the subsystems at any of the point codes in the PSTN.
FIG. 1 illustrates the interconnection of various components of the PSTN and the SS7 system involved in a 911 call from a mobile phone 10. The radio signal from mobile phone 10 is received by a base station controller, BSC, 12. The SS7 control signals from BSC 12 are coupled through one or more signaling transfer points, such as STP 14, to a mobile switching center, MSC, 16. When the MSC 16 recognizes that the call is a 911 call, it forwards the SS7 message through STP 18 to service control point, SCP, 20 which includes a 911 subsystem 22. The 911 subsystem 22 performs various functions, including sending caller identification and, if available, location information to the police end point, and sending a message back through STP 18 to MSC 16 to connect the voice channel to the police department 24. The 911 subsystem 22 also notes that the call is from a mobile phone and sends a SS7 message over the SS7 network to a position determination entity, PDE, 26 also contained in SCP 20. The PDE 26 may then obtain information from the sending telephone 10 by exchanging SS7 messages through STP 18, MSC 16, STP 14 and BSC 12 and calculate the location of phone 10. If the PDE 26 can locate the phone 10, it sends an SS7 message with the location information to the 911 service application 22 which then forwards the information to the police department. In FIG. 1, a second SCP 28 is illustrated connected to the system through STP 14. The PDE subsystem could be located in SCP 28 instead of SCP 20. Since the message from 911 service 22 to the PDE service is sent over the SS7 network, it will get to the PDE service regardless of which SCP contains the service. The SCP 28 may also contain one or more other subsystems.
When multiple applications are needed for a single call, the messages from one subsystem to another are sent over the SS7 system using the same protocol as all other SS7 messages. While this is a natural use of the SS7 system, it has several problems. For example, when a 911 application determines that the call originated from a mobile phone and that a message needs to be sent to a PDE, the SS7 message will have been partially processed. The standard processing of an SS7 message involves stripping off and processing one layer of the SS7 message stack at a time. By the time the processor gets to the point in the stack where it needs to contact another subsystem, it no longer has a complete SS7 message and must create a new one to be able to communicate with the other subsystem.
As noted above, multiple subsystems may reside at the same INS having the same point code. For example, the 911 and the PDE subsystems may both be located at the same point code, for example SCP 20 of FIG. 1. When the 911 subsystem 22 sends the message to the PDE subsystem 26, it may travel through one or more signaling transfer points 18 to a switch 16 and back through one or more signaling transfer points 18 to arrive at the same SCP 20 from which it was sent. It would be desirable to provide a more efficient system and methods for sending SS7 messages between subsystems on the PSTN.